I haven't been there on either day, but if I were to venture a guess I'd say July 4th.
One, it's the busy season anyway, right in the middle of summer. Two, while the 4th is a national holiday and families like to get together, it's not quite the extended family holiday that Christmas day is, I think. But then again, not everyone in the country celebrates Christmas while everyone in the country celebrate the 4th (since it's non-religious in nature). So that might even it out.
I'm sure both are absolutely insane and it probably isn't all that different, really.